The majority of them stick to Fusha, with maybe some very few (small) words expressed in dialect here and there. Just as a "slip of the tongue".
You do have scholars however that consciously decide to speak in dialect, although they are quite rare in my opinion. They might choose to do it to get as close to the laymen as possible by speaking the "living language". But speaking Fusha is not limited to religious scholars only...often it is for the educated ones like doctors, lawyers etc...you'll also find that in many political discussions on television by experts alot of fusha is spoken.